1. Field of the Invention
The present invention relates to an aqueous dispersion that contains a particle containing a water-insoluble colorant in a water-containing medium in a dispersed state and a preparation process thereof, and a particle that contains a water-insoluble colorant and has good dispersibility in water and a preparation process thereof.
2. Related Art
An ink-jet recording method is a method in which ink droplets are formed by any of various ink-ejecting systems, and a part or the whole of them are applied to a recording medium such as paper, processed paper, plastic film, fabric or the like, thereby conducting recording. Aqueous recording liquids composed mainly of a dye and water-soluble additives have heretofore been used as recording liquids for ink-jet. Although they are excellent in shelf stability as ink, those obtained by printing with these recording liquids may not have satisfactory properties in water fastness, light fastness or the like depending on uses thereof in some cases, and so a further improvement is being contemplated. It is thus attempted to improve the water fastness and light fastness by changing a coloring material from a dye to a pigment. However, a pigment ink is often poor in ejection stability from a nozzle of an ink-jet head compared with the dye ink. In addition, since the pigment is not a single coloring matter molecule such as a dye, but is composed of particles, an absorption spectrum becomes broad due to scattered light and reflected light by the pigment compared with the dye, so that the coloring of an image formed by a pigment ink generally tends to become low compared with that of an image formed by a dye ink. A method for solving the problems involving such coloring property includes such a method that the pigment is finely ground. There is thus a demand for finely grinding the pigment to 100 nm or smaller so as to lessen the influence of light-scattering and exhibit transmission property comparable to that of the dye. The fine grinding of the pigment is generally conducted by mechanical force using a dispersing machine such as a sand mill, a roll mill or a ball mill. According to this method, however, the fine grinding of the pigment is limited to about 100 nm to the degree of primary particles thereof. Accordingly, this method is difficult to be applied to a case where further grinding is required (Japanese Patent Application Laid-Open No. 10-110111). It takes a longer time for dispersion as the particle diameter of the pigment is more lessened, so that the cost is increased to a greater extent, and moreover difficulty is encountered on the provision of a pigment having uniform quality. Since it is very difficult to grind the pigment to smaller particles than the primary particles by this method, a pigment the primary particle of which is large cannot be used as a raw material for the purpose of providing pigment particles having a particle diameter smaller than 100 nm. On the other hand, there have been proposed processes that a pigment is dissolved once and deposited again to produce fine particles of the pigment. Japanese Patent Application Laid-Open No. 9-221616 has proposed fine grinding by the acid pasting process that an organic pigment is dissolved with sulfuric acid once. However, this process fails to provide pigment particles having a particle diameter smaller than 100 nm. Japanese Patent Publication Nos. 4-29707 and 6-4776 describe such a process that an organic pigment is dissolved in a non-protic polar solvent in the presence of an alkali and then neutralized with an acid to provide fine pigment particles. Since the fine grinding of the pigment and a dispersion-stabilizing treatment are not conducted at the same time in this process, however, pigment particles finely ground at first already undergo aggregation upon dispersion, and so it is difficult to provide a pigment dispersion of the substantially nanometer order. In Japanese Patent Publication Nos. 5-27664, 6-33353 and 6-96679, and Japanese Patent Application Laid-Open No. 11-130974, fine pigment particles are provided by dissolving an organic pigment and a dispersing agent such as a surfactant or a resin together in a non-protic polar solvent in the presence of an alkali and then neutralizing the solution with an acid to deposit the pigment. However, an investigation by the present inventors has revealed that the pigment particles obtained by this process are not sufficient in dispersion stability in an aqueous solvent containing water for applying them to, for example, a water-based ink for ink-jet. Since this process comprises a step of adding dropwise the acid to a pigment solution (water content: 20% or lower) to deposit the pigment, and separation of the pigment from the solvent by neutralization and reprecipitation is also conducted at the same time, it is impossible to sufficiently prevent association of the pigment particles, and so it is inferred that a pigment dispersion of the nanometer order having an even particle diameter cannot be stably provided even when a dispersing treatment using a ball mill or the like is subsequently conducted.
By the way, when a pigment-containing ink is used for ink-jet, it is considered that an ink excellent in shelf stability is obtained by finely grinding pigment particles, and a change in viscosity of the ink and a change in particle diameter of the pigment dispersion are little even when the ink is not used for a long period of time, and so good ejection property is easily achieved viewed from properties of terminal sedimentation velocity of the ink when the ejection property of the ink from a nozzle (ejection orifice) of an ink-jet head is considered. However, the finer the pigment particle, the more increase in the surface area per unit mass of the particle. Thus, the conventional way of a dispersion stabilization may not be enough for a dispersion comprising a particle having a diameter of nanometer order.
With respect to a pigment coated with a resin by only physical adsorption force, it leaves a fear of solvent resistance. When a pigment of such a structure is used as a component of an ink for ink-jet, in some cases, the pigment may form not only the cause that the kinds of various additives added to the ink are limited, but also the cause that the quality of the resulting image is deteriorated because a change in viscosity of the ink depending on temperature is liable to become great, and a quantity of the ink ejected from a nozzle of an ink-jet head varies.